In the field of consumer electronics there is a constant demand for new technologies and innovative products. An example here is reproducing audio signals as realistically as possible.
Methods of multichannel loudspeaker reproduction of audio signals have been known and standardized for many years. All conventional technologies have the disadvantage that both the positions of the loudspeakers and the locations of the listeners are already impressed onto the transmission format. If the loudspeakers are arranged incorrectly with regard to the listener, the audio quality will decrease significantly. Optimum sound is only possible within a small part of the reproduction space, the so-called sweet spot.
An improved natural spatial impression and increased envelopment in audio reproduction may be achieved with the aid of a new technique. The basics of said technique, so-called wave field synthesis (WFS), were investigated at the Technical University of Delft and were presented for the first time in the late 1980s (Berkhout, A. J.; de Vries, D.; Vogel, P.: Acoustic Control By Wavefield Synthesis. JASA 93, 1993).
As a result of the enormous requirements said method has placed upon computer performance and transmission rates, wave field synthesis has only been rarely used in practice up to now. It is only the progress made in the fields of microprocessor technology and audio coding that by now allow said technique to be used in specific applications.
The fundamental idea of WFS is based on applying Huygen's principle of wave theory: each point that is hit by a wave is a starting point of an elementary wave, which propagates in the shape of a sphere or a circle.
When applied to acoustics, any sound field may be replicated by using a large number of loudspeakers arranged adjacently to one another (a so-called loudspeaker array). To this end the audio signal of each loudspeaker is generated from the audio signal of the source by applying a so-called WFS operator. In the simplest case, e.g., when reproducing a point source and a linear loudspeaker array, the WFS operator will correspond to amplitude scaling and to a time delay of the input signal. Application of said amplitude scaling and time delay will be referred to as scale & delay below.
In the case of a single point source to be reproduced and a linear arrangement of the loudspeakers, a time delay and amplitude scaling may be applied to the audio signal of each loudspeaker so that the emitted sound fields of the individual loudspeakers will superpose correctly. In the event of several sound sources, the contribution to each loudspeaker will be calculated separately for each source, and the resulting signals will be added. If the sources to be reproduced are located in a room having reflecting walls, reflections will also have to be reproduced as additional sources via the loudspeaker array. The effort in terms of calculation will therefore highly depend on the number of sound sources, the reflection properties of the recording room, and on the number of loudspeakers.
The advantage of this technique consists, in particular, in that a natural spatial sound impression is possible across a large part of the reproduction room. Unlike the known technologies, the direction and distance of sound sources are reproduced in a highly exact manner. To a limited extent, virtual sound sources may even be positioned between the real loudspeaker array and the listener.
Application of wave field synthesis provides good results if the preconditions assumed in theory such as ideal loudspeaker characteristics, regular, unbroken loudspeaker arrays, or free-field conditions for sound propagation are at least approximately met. In practice, however, said conditions are frequently not met, e.g. due to incomplete loudspeaker arrays or a significant influence of the acoustics of a room.
An environmental condition can be described by the impulse response of the environment.
This will be set forth in more detail by means of the following example. It shall be assumed that a loudspeaker emits a sound signal against a wall, the reflection of which is undesired.
For this simple example, room compensation while using wave field synthesis would consist in initially determining the reflection of said wall in order to find out when a sound signal which has been reflected by the wall arrives back at the loudspeaker, and which amplitude this reflected sound signal has. If the reflection by this wall is undesired, wave field synthesis offers the possibility of eliminating the reflection by this wall by impressing upon the loudspeaker—in addition to the original audio signal—a signal that is opposite in phase to the reflection signal and has a corresponding amplitude, so that the forward compensation wave cancels the reflection wave such that the reflection by this wall is eliminated in the environment contemplated. This may be effected in that initially, the impulse response of the environment is calculated, and the nature and position of the wall is determined on the basis of the impulse response of this environment. This involves representing the sound that is reflected by the wall by means of an additional WFS sound source, a so-called mirror sound source, the signal of which is generated from the original source signal by means of filtering and delay.
If the impulse response of this environment is measured, and if the compensation signal that is superposed onto the audio signal and impressed onto the loudspeaker is subsequently calculated, cancellation of the reflection by this wall will occur such that a listener in this environment will have the impression that this wall does not exist at all.
However, what is decisive for optimum compensation of the reflected wave is the impulse response of the room is accurately determined, so that no overcompensation or undercompensation occurs.
Thus, wave field synthesis enables correct mapping of virtual sound sources across a large reproduction area. At the same time, it offers to the sound mixer and the sound engineer a new technical and creative potential in generating even complex soundscapes. Wave field synthesis as was developed at the Technical University of Delft at the end of the 1980s represents a holographic approach to sound reproduction. The Kirchhoff-Helmholtz integral serves as the basis for this. Said integral states that any sound fields within a closed volume may be generated by means of distributing monopole and dipole sound sources (loudspeaker arrays) on the surface of said volume.
In wave field synthesis, a synthesis signal is calculated, from an audio signal emitting a virtual source at a virtual position, for each loudspeaker of the loudspeaker array, the synthesis signals having such amplitudes and delays that a wave resulting from the superposition of the individual sound waves output by the loudspeakers existing within the loudspeaker array corresponds to the wave that would result from the virtual source at the virtual position if said virtual source at the virtual position were a real source having a real position.
Typically, several virtual sources are present at different virtual positions. The synthesis signals are calculated for each virtual source at each virtual position, so that typically, a virtual source results in synthesis signals for several loudspeakers. From the point of view of one loudspeaker, said loudspeaker will thus receive several synthesis signals stemming from different virtual sources. Superposition of said sources, which is possible due to the linear superposition principle, will then yield the reproduction signal actually emitted by the loudspeaker.
The possibilities of wave field synthesis may be exhausted all the more, the larger the size of the loudspeaker arrays, i.e. the larger the number of individual loudspeakers provided. However, this also results in an increase in the computing performance that a wave field synthesis unit supplies since, typically, channel information is also taken into account. Specifically, this means that in principle, a dedicated transmission channel exists from each virtual source to each loudspeaker, and that in principle, the case may exist where each virtual source leads to a synthesis signal for each loudspeaker, and/or that each loudspeaker obtains a number of synthesis signals which is equal to the number of virtual sources.
If the possibilities of wave field synthesis are to be exhausted, specifically, in cinema applications to the effect that the virtual sources can also be movable, it has to be noted that quite substantial computing operations have to be effected because of the calculation of the synthesis signals, the calculation of the channel information, and the generation of the reproduction signals by combining the channel information and the synthesis signals.
A further important expansion of wave field synthesis consists in reproducing virtual sound sources with complex, frequency-dependent directional characteristics. For each source/loudspeaker combination, convolution of the input signal by means of a specific filter is also taken into account in addition to a delay, which will then typically exceed the computing expenditure in existing systems.